1. Field of the Invention
The present invention relates generally to scanned-probe microscopes.
2. Description of the Related Art
The resolution of traditional optical microscopes that use lenses to focus radiation, e.g., visible light, is limited by diffraction to approximately 1/2 the radiation wavelength (a limit known as the "Abbe barrier" because of its description by Ersnt Abbe in the late 1800s). However, other imaging techniques are available to resolve details in the region below 250 nanometers, e.g., structures as small as single molecules, atomic scale crystal flaws, and microelectronic circuit patterns that have dimensions on the order of a few atoms.
Many of these techniques are complex and limited in their application. For example, an electron microscope typically includes an electron beam source, electron lenses, power supplies, and a vacuum chamber. Special sample preparation, e.g., metallization, is often required and the high-energy electrons may damage the sample. The art of X-ray diffraction typically requires an X-ray source, an X-ray collimator, a vacuum chamber and film exposure structures and is generally restricted to the investigation of atomic crystal lattices.
However, atomic scale topography of samples can be resolved by another, relatively simple, class of microscopes which are generally known as scanned-probe microscopes. These microscopes are characterized by a probe tip that is brought into close proximity with the surface of a sample to generate a measurable tip-to-surface interaction. In many scanned-probe microscopes, the proximity is so close that the electron clouds of the tip and sample atoms physically interface with each other.
For example, in a scanned tunneling microscope (STM), an electrical potential is imposed across the tip-sample gap. The potential causes tip and surface electrons to form a "tunneling current". This current is a measurable tip-to-surface interaction whose magnitude is extremely sensitive to the gap dimension. In an atomic force microscope (AFM), the force between the atoms of the tip and sample causes a deflection in a resilient cantilever that carries the tip; in this way the force becomes a measurable tip-to-surface interaction.
Sometimes the capillary action of a water film on the tip and/or sample can exert a tip-to-sample force that masks the atomic force. Accordingly, some scanned-probe microscopes immerse the tip-sample gap in a fluid, e.g., as described in U.S. Pat. No. Re. 34,489.
Other scanned-probe microscopes detect tip-to-surface interactions by vibrating the resilient cantilever near its resonant frequency (e.g., with a piezoelectric transducer). Any force that is exerted on the tip shifts the cantilever's resonant frequency; as a consequence, the vibration amplitude diminishes. The vibration amplitude then forms a measure of the tip-to-surface interaction.
The vibrating cantilever technique is particularly suited to microscopes in which the probe tip is spaced sufficiently from the sample to preclude the tunneling current effect and the electron cloud repulsion force but close enough to detect attractive forces. These include the surface tension of water that condenses between the tip and sample and the van der Walls interaction (relatively weak forces of attraction between atoms or molecules that are not bound to each other).
Scanned-probe microscopes can be used to measure sample properties other than their topography. The magnetic-force microscope (MFM) carries a magnetized nickel or iron probe on a vibrated resilient cantilever. The deflection amplitude can indicate the strength of magnetic field patterns, e.g., the pattern of a data-recording head. In an electrostatic-force microscope (EFM), a potential across the tip-sample gap has been used to map electrostatic forces, e.g., dopant patterns in semiconductors. A scanning thermal microscope has a tip that includes a thermocouple junction of two metals, e.g., tungsten and nickel. After this tip is heated, its heat loss varies with the tip-sample gap dimension. The voltage output from the thermocouple is a measurable tip-to-surface interaction. Further exemplary scanned-probe microscopes are described by Wickramasinghe (see Wickramasinghe, H. Kumar, "Scanned-Probe Microscopes", Scientific American, October, 1989).
Scanned-probe microscope systems are conventionally characterized by a feedback control circuit which compares an output signal that is related to the tip-to-surface interaction, with a predetermined reference to develop an error signal. The error signal is processed to form a control signal which is applied to a translation device, typically a piezoelectric transducer, that moves the tip and/or sample to control the tip-to-surface interaction. The sample surface is generally defined relative to an orthogonal x, y, z coordinate system. It is customary to align the x, y coordinates with the sample plane and define the tip-sample gap with the z coordinate.
U.S. Pat. No. Re. 34,331 is directed to a feedback control system for scanned-probe microscopes. As described in the Patent, "--the enhanced feedback--takes information from stored previous knowledge of the scan of the sample and specifically, information regarding the previous knowledge of the topography of the sample, and includes that information as part of the feedback control loop for the scan tip so that the tip can better follow the surface topography of the sample at each present location. Anticipating the topography thereby allows the system to provide a better following of the topography by not relying solely on the local error signal to adjust or change the height of the tip. The anticipation thereby uses the topographical information to increase the response of the feedback loop and also to provide for a more accurate rendering of the topography of the object under investigation by the scanning tunneling microscope." Digital and analog implementations are described. The digital implementation includes a feedback control in which the tunneling current is digitized by an A/D converter. The digitized tunneling current is then applied to a computer. The computer calculates what the vertical position of the tip should be and this calculation is supplied via a D/A converter to produce the proper vertical positioning signal, i.e., the control signal. The computer can also be used to accomplish integral, proportional and differential feedback.
The measurement scale of scanned-probe microscopes is extremely small. An exemplary x, y scan area may be 20.times.20 nanometers with a z-dimension movement on the order of 2 nanometers. Preparing and carrying out surface measurements in such a minute scale can be time consuming. Accordingly, U.S. Pat. No. Re. 34,489 is directed to setup and alignment problems of the sample and probe tip. In particular, positioning problems that involve the sample, the tip, a laser-emitting diode and a photoelectric sensor are addressed.
Adjusting and choosing a microscope system's operational characteristics for the investigation of a sample surface can also be a time-consuming effort. Analog control loops have features, e.g., excellent performance, simplicity and relatively low cost, that make them particularly suited for use in scanned-probe microscope systems. However, the topographical data of a surface is quite sensitive to the analog loop characteristics and considerable time is typically spent in adjusting these characteristics until acceptable quality of topographical data is obtained.